London  - A group of EU lawmakers on Monday threatened to veto Britain's Brexit agreement unless the government improves its offer to protect EU citizens.

Prime Minister Theresa May's offer to protect EU citizens resident in Britain "would cast a dark cloud of vagueness and uncertainty over the lives of millions of Europeans," Guy Verhofstadt, the European Parliament's lead negotiator for Brexit, and eight other cross-party lawmakers wrote in The Guardian.

May offered to protect the "settled status" of EU citizens, vowing that no families would be separated.

She promised the families of EU citizens rights broadly in line with those of non-EU family members of British citizens, including settled status after five years.

No for "second-class citizenship"

The lawmakers said the gap was "striking" between May's offer and a proposal by the EU’s Brexit negotiator, Michel Barnier.

"Barnier wants British people and Europeans to keep the same rights and the same level of protection they currently enjoy under European law," they wrote. But Britain's offer could leave EU citizens in Britain with "fewer rights than British citizens are offered throughout the EU."

"Europeans will not only lose their right to vote in local elections, but family members will be subject to minimum income requirements, and it is unclear what the status of 'post-Brexit' babies would be," they said.

"This carries a real risk of creating second-class citizenship."

They warned that the European Parliament has the right to "reject any agreement that treats EU citizens, regardless of their nationality, less favourably than they are [being treated] at present."